1. Technical Field
The present disclosure relates to the electric connection between an integrated circuit pad and a copper wire.
2. Description of the Related Art
A connection mode between an integrated circuit pad and a connection wire is shown in FIGS. 1A and 1B.
FIG. 1A shows a portion of an integrated circuit 1 comprising a connection pad 2 to which a copper wire 3, comprising, at its end intended to be attached to the pad, a ball 4, is desired to be welded or bonded. Pad 2 is formed of a stack of three layers, that is, a copper layer 5, a titanium nitride barrier layer 6, and an aluminum layer 7. A passivation dielectric layer 8 covers the periphery of the welding pad and defines a window 9 which delimits surface 10 of the welding pad.
The welding is performed between the end of the wire comprising the copper ball and aluminum layer 7 heated between 150° and 180° C. The copper ball is pressed against the pad and ultrasounds are applied thereto.
FIG. 1B shows the structure after welding. Ball 4 is crushed against surface 10 of pad 2. Ball 4 penetrates into the aluminum layer. The aluminum pushed back by the ball forms extrusions 21 and a space 20 forms between the copper ball periphery and surface 10. Contact areas 22 between the copper ball periphery and surface 10 are exposed to the ambient air which flows in space 20.
FIG. 2A illustrates the copper aluminum compounds present between layer 7 and ball 4. From aluminum layer 7, aluminum-rich compounds 23 such as CuAl2, followed by components comprising less aluminum 24, such as CuAl, and finally, at the interface with copper ball 4, copper-rich compounds 25 such as Cu9Al4 or Cu3Al2, can be found.
By reaching areas 22, the humidity of the ambient air and of the chlorine present in the materials or brought from the outside may modify the chemical composition of copper aluminum compounds.
FIG. 2B illustrates the aluminum-oxygen compounds resulting from this transformation, such as Al2O3 or aluminum-chlorine, such as AlCl2, forming layer 26 located between layer 23 and layer 4. The copper aggregates in islands in layer 26.
Stress resulting from the integrated circuit operation, like vibrations or temperature variations, weakens the interface layers located in area 22, and a separation between the pad and the copper wire can sometimes be observed during the circuit lifetime, despite the optimizations of welding techniques.
A reliability issue linked to the welding of the copper wire on aluminum is thus raised, this issue becoming more critical during the use of the product.